A refractive optical element acts upon a wavefront that is transmitted through the element. To ensure proper functioning of the optical element or as part of the manufacturing process, a measurement of the transmitted wavefront through an optical element may be performed. One way to directly measure the transmitted wavefront is to use an interferometer, and many different configurations of interferometers are described in prior art. In order to measure the transmitted wavefront of an optic, one usually aligns the unit under test (UUT) such that it is part of a null optical configuration. There are many null configurations that are described in the prior art such as in Malacara, Daniel, (ed.), Optical Shop Testing, Second Edition, John Wiley & Sons, Inc., 1992. One class of optics that has proven difficult to test is dome-like optics.
The surfaces, not the transmitted wavefront, of dome-like optics have been tested in various configurations. For instance in Simpson et al, a “Hindle shell test” is described (F. A. Simpson, et al, “Testing Convex Aspheric Lens Surfaces with a Modified Hindle Arrangement,” Opt. Eng., 13, G101 (1974)). In the configuration described, a conventional interferometer, such as a Fizeau type, is used to measure the quality of a single surface of a lens, not the transmitted wavefront of the Hindle shell. In particular, a partially reflective coating is applied to the inner surface of the Hindle shell and an interferometer is used to compare the single surface of the lens with the interferometer reference surface. While not described in the paper, one could use this approach to test a surface of the Hindle shell if the partially reflective coating is applied. Adding a coating to the UUT that must subsequently be removed is an undesirable step in the manufacturing process.
One reason that it is difficult to test dome-like optics is that the surfaces are typically concentric, making it difficult to separate light reflected by one surface from the light reflected by the other. A conventional interferometer measures the shape of a surface by comparing (i.e., interfering) the light reflected from a single surface with the light reflected by a reference surface. However, the concentric surfaces of a dome-like optic result in multiple beam interference and erroneous results. This makes calculating the figure of the surface difficult. It is possible to index match one surface of the dome-like optic so that one reflection is suppressed, but this adds additional steps to the testing process.
Another reason why dome-like optics are difficult to test is that they typically subtend a large portion of a hemisphere. The numerical aperture of interferometer test optics can only capture a sub-aperture in each measurement. One can then hope that if the sub-apertures are of adequate quality, then the entire dome is of adequate quality. Alternatively, additional data processing complexity can be added to assemble a full surface map from the sub-aperture data, which is often referred to as stitching. This is a time-consuming process and the motion of the dome-like optic between measurements can add error.
An even more challenging problem is the measurement of a dome-like optic when the index of refraction varies. When this is the case, measuring the shape of the two surfaces of the dome-like optic does not ensure that the transmitted wavefront through the dome-like optic is of adequate quality. This means that direct measurement of the transmitted wavefront is required. For a dome-like optic that subtends a large angle, the current measurement approach is to use sub-aperture measurements. When this is done, errors due to misalignments between the interferometer, null optic (if used) and the UUT and the fabrication errors in the optic cannot be easily distinguished, if at all.
What is required is an approach for measuring the transmitted wavefront of a dome or dome-like optic where the full aperture of the optic can be measured simultaneously. It is also desirable to have an approach that will work with optics that are not spherical and whose thickness or index of refraction varies with position.